RFR: 8295278: Add parallel class loading tests [v2]

[Coleen Phillimore]

> I moved the ParallelLoad test to a subdirectory, and created a new one called CallLoadClass that calls loadClass directly and then waits on the ClassLoader object, while another thread calls forName() to go through the SystemDictionary resolution.
> 
> I also made dedicated tests out of the bugs reported in
> [JDK-4699981](https://bugs.openjdk.org/browse/JDK-4699981) ClassCircularityError thrown without reason during class loading and
> [JDK-4670071](https://bugs.openjdk.org/browse/JDK-4670071) java.lang.ClassLoader.loadClassInternal(String) is too restrictive
> 
> Tested with tier1 on Oracle platforms. These tests are run in tier1.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Fix success test for loadClass case.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10587/files
  - new: https://git.openjdk.org/jdk/pull/10587/files/abab0205..796c7b45

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10587&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10587&range=00-01

  Stats: 5 lines in 1 file changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10587.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10587/head:pull/10587

PR: https://git.openjdk.org/jdk/pull/10587